Nonlinearities in the gain of a power amplifier are considered undesirable because intermodulation distortion (IMD) is generated for input signal formats with varying amplitude. Predistortion is a method of compensating for a power amplifier with nonlinear gain by producing an inverse gain in an attempt to make the combined gain linear. When the system is linear, the overall gain is constant for any input power level. The variance of the gain over the range of input power levels is a measure of the nonlinearity of the system.
A predistortion module preceding the main amplifier reduces output distortion only when tuned properly. In order to tune the predistortion adaptively, it is necessary to measure the input and output of the main amplifier and estimate the inverse gain characteristics. If the input and output samples are obtained from log-detectors, the differential log measurement provides information regarding the gain. Unfortunately, the differential log measurements are not reliable because outlier measurements with large errors are possible. Such outlier measurements corrupt the search process for the optimal tuning parameters.
Accordingly a need presently exists for a predistortion approach which can address the above noted problems and provide more reliable adaptive predistortion for amplifier linearization.